G-API: "deownification" corrections

- moved "standalone" aliases of cv types to "opencv_includes.hpp" to
keep responsibility principle aplied
- introduced correct aliasing for "descr_of(Mat)" function for
standalone case
 - added missing include in `gapi/own/mat.hpp`

modules/gapi/src/api/gmat.cpp

@@ -48,9 +48,9 @@ namespace{
     }
 }
 
+#if !defined(GAPI_STANDALONE)
 cv::GMatDesc cv::descr_of(const cv::Mat &mat)
 {
-#if !defined(GAPI_STANDALONE)
     const auto mat_dims = mat.size.dims();
 
     if (mat_dims == 2)
@@ -62,12 +62,8 @@ cv::GMatDesc cv::descr_of(const cv::Mat &mat)
         dims[i] = mat.size[i];
     }
     return GMatDesc{mat.depth(), std::move(dims)};
-#else
-    return (mat.dims.empty())
-        ? GMatDesc{mat.depth(), mat.channels(), {mat.cols, mat.rows}}
-        : GMatDesc{mat.depth(), mat.dims};
-#endif
 }
+#endif
 
 cv::GMatDesc cv::gapi::own::descr_of(const Mat &mat)
 {

modules/gapi/src/api/gscalar.cpp

@@ -46,6 +46,8 @@ const cv::GOrigin& cv::GScalar::priv() const
     return *m_priv;
 }
 
+//N.B. if we ever need more complicated logic for desc_of(cv::(gapi::own::)Scalar)
+//dispatching should be done in the same way as for cv::(gapi::own)::Mat
 cv::GScalarDesc cv::descr_of(const cv::Scalar &)
 {
     return empty_scalar_desc();